1. Field of the Invention
The present invention relates to a plasma treatment apparatus, which can be used for removing foreign substances such as organic substances from objects to be treated, stripping resists, improving adhesion of organic films, surface modification, film formation, reducing metal oxides, or cleaning glass substrates for liquid crystal, and also a plasma treatment method performed by use of the same apparatus, which can be applied to clean surfaces of electronic parts requiring precise connections.
2. Disclosure of the Prior Art
In the past, surface treatments have been performed to substrates by use of plasma (glow discharge) stably generated under atmospheric pressure. For example, Japanese Patent Early Publication [KOKAI] No. 2-15171, No. 3-241739 or No. 1-306569 discloses a plasma treatment, which is characterized by arranging a pair of electrodes in a discharge space in a reaction vessel, disposing a dielectric material between the electrodes, filling the discharge space with a gas for plasma generation mainly composed of an inert gas such as He (helium) or Ar (argon), applying an AC voltage between the electrodes to generate plasma of the gas for plasma generation, and treating an object put in the reaction vessel with this plasma.
However, there are problems that it is difficult to perform the plasma treatment to only a selected area of the object, and the treatment time is too long. On this account, it is proposed to perform a plasma treatment by use of a plasma jet (particularly, radicals in plasma) generated by glow discharge under atmospheric pressure. For example, various kinds of methods are disclosed in Japanese Patent Early Publications [KOKAI] No. 4-358076, No. 3-219082, No. 4-212253 and No. 6-108257.
In each of the methods disclosed in Japanese Patent Early Publications [KOKAI] No. 3-219082, No. 4-212253 and No. 6-108257, a plasma jet is supplied from a nozzle-like reaction tube toward objects to be treated. However, this method still has room for improvement with respect to the following points.
(1) Since the treatment range is small, it is not suitable to treat large area.
(2) When the discharge space is narrowed, heat radiation properties lower and the interior of the reaction tube is heated at a high temperature, so that the objects to be treated easily suffer thermal damage. On the other hand, when the discharge space is enlarged, the treatment efficiency lowers.
(3) When the interior of the reaction tube is heated at such a high temperature, streamer discharge (arc discharge) easily occurs between the electrodes or between the reaction tube and the object. Therefore, it is not suitable to stably provide a uniform plasma treatment.
In addition, Japanese Patent Early Publication [KOKAI] No. 4-358076 discloses a plasma treatment apparatus having dielectric plates disposed between flat-panel electrodes. In this apparatus, there are problems that dimensions of the apparatus are large, and the treatment speed is slow because it is difficult to obtain a high plasma density from the reasons described below. That is, due to a large area of the flat-panel electrode, an electric power per unit volume of plasma lowers. By applying a larger electric power, the plasma density can be increased. However, the electrode is heated at a high temperature, so that the objects to be treated may suffer thermal damage, or a breakage of the electrodes or streamer discharge may occur. Moreover, in a method of putting the dielectric plate on the flat-panel electrode, it is difficult to reduce a thickness of the dielectric plate because of structural problems. As the structural problems, for example, when putting a glass plate as the dielectric plate on the electrode, it is difficult to adhere the glass plate having a thickness of less than 1 mm to the flat-panel electrode over the large area due to a poor strength of the glass plate. In addition, when using a ceramic plate as the dielectric plate, it is possible to produce the ceramic plate having a thin thickness and a sufficient strength. However, it is difficult to produce the ceramic plate having a large area corresponding to the flat-panel electrode, and also obtain good adhesion with the electrode material. Consequently, since a voltage drop is caused at the dielectric plate, it becomes difficult to increase the electric power per unit volume of plasma.
In view of the above points, a concern of the present invention is to provide a plasma treatment apparatus capable of efficiently performing a plasma treatment to a large area of an object placed at downstream from a discharge space, while preventing the occurrence of streamer discharge. The plasma treatment apparatus of the present invention comprises at least one pair of electrodes, a gas supply unit for supplying a gas for plasma generation to the discharge space defined between the electrodes, and a power supply for applying an AC voltage between the electrodes to generate plasma of the gas for plasma generation in the discharge space. At least one of the pair of electrodes has a dielectric layer at an outer surface thereof. In the present invention, at least one of the pair of electrodes has a curved surface jutting into the discharge space.
Using the electrode having the curved surface jutting into the discharge space is effective to increase a plasma density with the use of a reduced electric power while preventing the occurrence of streamer discharge. That is, when using an electrode having a sharp-pointed edge portion jutting into the discharge space, the plasma density can be increased. However, the streamer discharge (arc discharge) easily occurs at such an edge portion according to unevenness of electric field. Once the streamer discharge occurs, the plasma treatment can not be continued because the plasma (glow discharge) becomes unstable. In addition, when the streamer discharge occurs between the electrode and the object, there is a problem that the object suffers considerable thermal damage. When using the electrode having the curved surface jutting into the discharge space, it is possible to increase the plasma density and prevent the streamer discharge. In particular, it is preferred that a radius of curvature of the curved surface is within a range of 1 to 25 mm.
It is preferred that a plasma guide member is disposed adjacent to at least one of the pair of electrodes to guide the plasma such that the plasma spreads out toward the object from the discharge space. In addition, it is preferred that the plasma guide member is integrally formed with the electrode. The plasma guide member is useful to efficiently perform the plasma treatment to a large area of the object.
It is further preferred that at least one of the pair of electrodes is a tubular electrode, and more preferably a cylindrical electrode. In this case, it is particularly preferred that the plasma treatment apparatus includes a coolant supply unit for supplying a coolant to the interior of the tubular or cylindrical electrode to reduce an electrode temperature during the plasma treatment. Reducing the electrode temperature is effective to prevent the streamer discharge, as explained later in detail.
A further concern of the present invention is to provide a plasma treatment method performed by use of the plasma treatment apparatus described above. That is, the plasma treatment method comprises the steps of supplying the gas for plasma generation to the discharge space between the electrodes, applying an AC voltage between the electrodes to generate plasma of the gas for plasma generation in the discharge space under atmospheric pressure, and treating the object with the plasma.
These and still other objects and advantages will become apparent from the following detail description of the invention and examples of the invention.